In a MIMO wireless communication system, a first device having a plurality of antennas sends multiple spatial stream transmissions to a second device having a plurality of antennas. When the number of spatial streams sent by the first device is less than the number of transmitters (and transmit antennas) of the first device, the spatial streams are expanded across transmitters of the first device in order to fully utilize the transmitter resources available at the first device.
In an IEEE 802.11n wireless communication system, the method of spatial expansion is not defined for high throughput (HT) transmissions; however spatial expansion is desired to fully utilize the transmitter resources. Knowledge of the spatial expansion method employed at the transmitting (first) device is desired at the receiving (second) device in order to improve the beamforming methods or adaptive rate selection performed at the second device for transmissions made to the first device.